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The State Services Commissioner has launched an investigation into chief executive and Secretary to the Treasury Gabriel Makhlouf over the Budget breach.
It comes after the National Party released documents it said was from the Budget, ahead of the Budget's release on May 30.
A statement from Commissioner Peter Hughes this afternoon said the investigation would establish the facts about Mr Makhlouf's public statements about the unauthorised access to Budget documents, his advice to the Finance Minister, his basis for them and his decision to refer the matter to police.
The questions raised were a "matter of considerable public interest and should be addressed," Mr Hughes said.
"Mr Makhlouf believes that at all times he acted in good faith.
"Nonetheless, he and I agree that it is in everyone's interests that the facts are established before he leaves his role on 27 June if possible. Mr Makhlouf is happy to co-operate fully to achieve that. I ask people to step back and let this process be completed."
Mr Makhlouf is leaving his role at the Treasury to take up a position as head of Ireland's central bank.
Deputy State Services Commissioner John Ombler, QSO, will lead the investigation.
Last week, the National Party released some details of the then-upcoming Budget.
Finance Minister Grant Robertson said some of the documents were correct and others were not.
A release from the Treasury that evening said its "systems [had] been deliberately and systematically hacked" and that it had referred the matter to police on the advice of the National Cyber Security Centre.
"The Treasury takes the security of all the information it holds extremely seriously. It has taken immediate steps today to increase the security of all Budget-related information and will be undertaking a full review of information security processes," it said.
The inquiry into Mr Makhlouf's actions will be separate from the inquiry into authorised access of Budget information.